64 ‘On the UNEQUAL 
‘M. Boscovicu formed an hypothefis concerning a perfec 
correction of colour, by a combination of mediums, which ap- 
pears to have greatly mifled him. Asa combination of two 
mediums is neceflary to unite two of the unequally refrangible 
rays, he imagines three mediums neceflary to unite three, four 
to unite four, and, in fhort, that to effect a perfect union of the 
rays of the fpectrum, as many mediums are required as there 
are unequally refrangible rays compofing it, that is to fay, an ~ 
indefinite number. He fuppofes, however, than’ an union of 
three of the rays only, by means of three mediums, would 
greatly improve telefcopes. This author feems to have founded 
his hypothefis on the fame kind of loofe analogical reafoning, 
which had before led the celebrated EULER into a fimilar mi- 
ftake. 
Tue eye is compofed of three humours and eared coats 3 
and M. Boscovicu takes it for granted, that a more perfec 
union of the rays than what takes place in a combination of 
crown and flint glafs, is effected by their means. But this is 
a fuppofition very remote from the truth indeed. ,, So far is this 
fecondary colour from being corrected in the human eye, that 
in the conftruction of this admirable organ, it hath been 
deemed unneceflary to introduce any contrivance for the cor- 
rection of the Newtonian aberration. Natura nihil agit fruftra. 
The perfection of the Contriver equally appears from a mani- 
feftation of his power, and of his ceconomical exertion of that 
power. On account of the fhortnefs of the focal diftance of 
the humours of the eye, in proportion to the aperture of the 
pupil, the aberration from the {pherical figure would be enor- 
mous ; and we find it obviated. by the very elaborate artifice of 
rendering the chryftalline humour more denfe towards the cen- 
tre. The aberration from difference of refrangibility might 
have been removed, by imparting a proper degree of difperfive 
power to the vitreous humour. But this, being unneceflary for 
the common purpofes of life, is withheld. 
Dr 
